Closing unit for injection molding machine

ABSTRACT

A closing unit for an injection moulding machine has a movable closing die and a fixed matching die designed as clamping dies for an injection mould, as well as several hydraulic cylinders mounted on the fixed matching die to apply a closing force. A servo-drive allows the drawbars to swivel around their longitudinal axes up to a first and second angular position. A first locking device arranged on the drawbars along a first section A of the drawbars co-operates with second locking device arranged on the movable closing die. In the first angular position of the drawbars, the locking devices allow the movable closing die to move axially with respect to the drawbars, and in a second angular position of the drawbars they transmit the required closing force.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a closing unit for an injection moldingmachine.

2. Related Art

The closing unit of an injection molding machine receives the injectionmold. It carries out the movements necessary for the closing and openingof the injection mold and produces the forces necessary for the lockingand opening of the injection mold. The main components of each closingunit are a stationary plate on the injection side (hereinafter referredto as the injection plate), a movable closure plate, as well as alocking device. One part of the injection mold is clamped on thestationary injection plate while the complementary part of the injectionmold is clamped on the movable closure plate. By locking device there isto be understood the device which, upon the injection and furtherpressing, produces the necessary closing force for keeping the injectionmold closed.

Both mechanical locking devices with lever mechanisms and hydrauliclocking devices with hydraulic cylinders are known.

The present invention relates to a closing unit with hydraulic locking.In the book “Kunstsfoff-Maschinen-Führer,” 3rd edition, edited by Dr.Eng. Friedrich Johannabe, published by Carl Hanser Verlag (Munich,Vienna), 1992 various constructions of closing units with hydrauliclocking are described.

In most closing units with hydraulic locking, a force cylinder isdeveloped as a pressure cushion and is arranged on a support platebehind the movable closure plate.

From FIG. 56, page 110, of said book, a closing unit having fourhydraulic cylinders on the stationary injection plate is, however,known. The pistons of these hydraulic cylinders are connected rigidlyvia connecting rods with the movable closure plate. The four hydrauliccylinders produce the required closing force but also, at the same time,carry out the opening and closing movements of the movable closureplate. Since the four hydraulic cylinders at the same time perform alocking function and a displacement function, they must be both of largecross section and have a large stroke and they therefore take up arelatively large amount of space. The four relatively large hydrauliccylinders accordingly substantially impede access to the injection unitand furthermore have a very high consumption of oil. The structurallength of the closing unit is substantially greater than the greatestpossible distance between the injection plate and the closure plate.

From FIG. 53B, page 108, of the same book, a more compact closing unit,also having four hydraulic cylinders on the stationary injection plate,is known. The four connecting rods are detachably connected by claws attheir ends to the corresponding hydraulic cylinder. Opening and closingmovements are produced, with connecting rods uncoupled, by high-speedcylinders arranged on the side. The stroke of the four hydrauliccylinders in this construction must accordingly correspond merely to thedifference in length between the largest and smallest injection molds.This construction is primarily of interest when the injection molds usedall have more or less the same length. If the closing unit, however, isto be used with injection molds of different length, the stroke of thehydraulic cylinders must be relatively large and the closing unit ofFIG. 53B has substantially the same disadvantages as the aforementionedclosing unit of FIG. 56.

The object of the present invention is to create a compact closing unitwhich is suitable for injection molds of different length. The object isachieved by a closing unit in accordance with claim 1.

SUMMARY OF THE INVENTION

The closing unit of the invention comprises, in known manner, a fixedinjection plate with injection opening and a movable closure plate,these plates forming, for instance, clamping plates for an injectionmold. A displacement device for the movable closure plate produces theopening and closing movements and makes it possible to position themovable closure plate relative to the fixed injection plate. Severalhydraulic force cylinders, preferably four, are arranged on thestationary injection plate and produce the closing force. This closingforce is transmitted by connecting rods, each of which is connectedmechanically with the piston of a force cylinder to the movable closureplate. The closing unit of the invention differs from the known closingunit, which has the features indicated above, primarily by the fact thatthe connecting rods are turnable around their longitudinal axes into afirst and a second angular position, first locking means being arrangedalong a rod section A on the connecting rods and second locking meansbeing arranged on the movable closure plate, and these first and secondlocking means being developed complementary to each other in such amanner that in the first angular position of the connecting rods theymake and axial displacement of the movable closure plate relative to theconnecting rods possible and that in the second angular position of theconnecting rods the first locking means in the rod section A on theconnecting rods cooperate with the second locking means on the movableclosure plate to transmit the necessary closing force.

The closing unit of the invention permits an extremely compact,space-saving construction. The required stroke of the force cylindersand thus their outside dimensions are minimum. As compared with knownclosing units, the closing unit of the invention is characterized by avery small structural length. The structural length of the closing unitneed in fact not be substantially larger than the greatest possibledistance between the injection plate and the closure plate. Thehydraulic connections for the force cylinders and the displacementdevice of the closure plate can be arranged on the fixed injectionplate. Flexible hydraulic connections are, accordingly, not required.Therefore the closing unit is excellently suited for receiving injectionmolds of different length. The range of length of the injection molds islimited here solely and exclusively by the length of the rod section Ahaving the first locking means.

The first locking means advantageously comprise an outer toothing on theconnecting rod, and the second locking means comprise a locking bushingwhich is fastened fixed for rotation on the movable closure plate andhas an inner toothing. Inner toothing and outer toothing are divided bylongitudinal grooves into at least two rows of teeth. In the firstangular position, the rows of teeth of the outer toothing can be passedthrough axially by longitudinal grooves of the inner toothing and therows of teeth of the inner toothing can be passed through axially by thelongitudinal grooves of the other toothing, so that an axialdisplacement of the movable closure plate relative to the connectingrods can take place. In the second angular position, the teeth of theinner toothing, on the other hand, can engage behind the teeth of theouter toothing for the transmission of a pulling force.

The teeth of the inner and outer toothings can be arranged annularly orhelically. In order to assure a dependable engagement of the outertoothing in the inner toothing, a substantially axial flank clearanceshould be developed between inner and outer toothings. A large flankclearance, however, also has substantial disadvantages. Thus, forexample, the operating stroke of the force cylinders is increasedthereby, and thus the consumption of energy by the closing unit. Theflows through the force cylinders are considerably greater so that thehydraulic system of the closing unit must also be made larger.Furthermore, the unlocked connecting rods are relatively stronglyaccelerated upon overcoming a large flank clearance, so that the teethof the inner toothing strike with great force against the teeth Of theouter toothing.

Within the scope of this invention, however, there is proposed anextremely simple and inexpensive solution which completely eliminatesthe disadvantages of a large axial flank clearance. This solutionconsists essentially therein that the piston of the hydraulic forcecylinder is coupled by a screw thread to the connecting rod in a verysimple manner and that the piston is secured against turning. Uponturning of the connecting rod by an angle γ from the first angularposition into the second angular position, the connecting rodaccordingly experiences an advance X relative to the fixed piston. Thepitch of the screw thread is then so designed that by turning theconnecting rod from the first angular position into the second angularposition, the existing axial flank clearance S between inner and outertoothings is distributed on one side in such a manner that nosubstantial flank clearance is present any more between the tooth flankswhich are to transmit the force. If, in this connection, the actuatorfor the turning of the connecting rods is so designed that it can placethe connecting rods into a second angular position both bycounterclockwise rotation and by clockwise rotation, then the flankclearance S between inner and outer toothings is distributed, dependingon the direction of rotation, on the one hand to the left side, and onthe other hand to the right side. The toothings are accordinglyautomatically without clearance for the transmission of the closingforce in the first direction of rotation and automatically withoutclearance for the transmission of an opening force to the closure platein the second direction of rotation.

The movable closure plate advantageously has rotatably mounted slideshoes as radial guide for the connecting rods. The longitudinal groovesin the outer toothing form guide surfaces for these slide shoes, whichare extended beyond the rod section A. The slide shoes are turnable withthe connecting rod.

One very advantageous embodiment of the actuator as well as a veryadvantageous hydraulic control of the closing unit will be describedinter alia in the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments, as well as various features and advantages, of theinvention will be described in detail with reference to the figures ofthe accompanying drawings, in which:

FIG. 1 is a view of closing unit in accordance with the invention;

FIG. 2 is a longitudinal section through the closing unit of FIG. 1;

FIGS. 3 and 4 are a cross section through a connecting rod and a lockingbushing of the closing unit of FIG. 1;

FIG. 5 is a cross section through the connecting rod;

FIG. 6 is a cross section through the locking bushing;

FIG. 7 is a section along the section line A—A of FIG. 3;

FIG. 8 is a section along the section line B—B of FIG. 4;

FIG. 9 is an enlargement from the longitudinal section of FIG. 2;

FIG. 10 is a section through an actuator for two connecting rods;

FIGS. 11 to 14 are cross sections through different embodiments of theconnecting rod;

FIGS. 15 to 20 are 180° developments of the toothings on the connectingrod and the locking bushing; and

FIG. 21 is a diagram of the hydraulic control of the closing unit.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

The general construction of a closing unit 10 in accordance with theinvention will be explained with reference to FIGS. 1 and 2. Aninjection plate 14 having a central injection opening 16 is rigidlymounted on a base 12. The base 12 forms a guide bed 18 for a movableclosure plate 20. The complementary halves of an injection mold (notshown) are clamped to injection plate 14 and closure plate 20. Themovable closure plate 20 is displaceable via an actuating device whichcomprises, for instance, two laterally arranged displacement cylinders25 movable between injection plate 14 and an end plate 24. It is guidedin this connection in the base 12. The displacement cylinders 25accordingly open and close the complementary halves of the injectionmold by displacement of the closure plate 20 relative to the injectionplate 14. The housing of the displacement cylinders 25 is fastened ineach case on the stationary injection plate 14 so that both displacementcylinders 25 have a rigid hydraulic connection on the fixed end plate14.

Four connecting rods 22 extend from the stationary injection plate 14through the movable closure plate 20 up to the end plate 24 which isalso mounted fixed on the base 12. In the end plate 24 connecting rods22 are guided in axially displaceable manner in slide bushings 32. Onthe stationary injection plate 14, there is associated with eachconnecting rod 22 a force cylinder 26 the housing of which is rigidlyconnected to the injection plate 14.

In FIG. 2 it can be seen that each of these force cylinders 26 comprisesan annular piston 28 which is mechanically attached to the connectingrod 22. A first pressure chamber 30 on the front side is axially limitedon the one hand by the injection plate 14 and on the other hand by thepiston 28. If, this first pressure chamber 30 is placed under pressure,then the piston 28 exerts an axial pulling force on the connecting rods22, whereby the force of reaction is taken up by the stationaryinjection plate 14. This first pressure chamber 30 produces the closingforce necessary for locking the injection mold. Within a second pressurechamber 31, the piston 28 forms a substantially smaller pressure surfaceas shoulder surface. If this second pressure chamber 31 is placed underpressure and the first pressure chamber relieved of pressure, then thepiston 28 exerts an axial pressing force on the connecting rods 22 inthe direction opposite the closing force described above. This pressingforce serves to open the mold after the casting.

In the movable closure plate, each connecting rod 22 passes through alocking bushing 34 having first locking means 36. Second locking means38, which are complementary to the first locking means, are providedalong a rod section A on the connecting rod 22. These first and secondlocking means 36 and 38 as so developed that in a first angular positionof the connecting rod 22, the latter can be passed through axially bythe locking bushing 34, that the second locking means, however, can bebrought by turning the connecting rod 22 into a second angular positionin which it cooperates within the rod section A with the first lockingmeans in order to transmit the necessary closing force.

For the closing of the injection mold by displacement of the movableclosure plate 20 by means of the displacement cylinders 25, the secondlocking means 38 are in the first angular position. In this firstangular position, the connecting rods 22 slide upon displacement of theclosure plate 20 axially through the locking bushings 34. Thereupon, theconnecting rods 22 are locking in the locking bushings 34 of the movableclosure plate 20 by turning from the first angular position into thesecond angular position. The four force cylinders 26 can now transmitthe required closing force via the connecting rods 22 to the closureplate 20, i.e. pull the closure plate 20 in the direction towards theinjection plate 14.

One advantageous embodiment of the locking means will be described infurther detail with reference to FIGS. 2 to 8. The locking means on theconnecting rod 22 advantageously comprise (see FIGS. 2 and 5) an outertoothing 40 which is divided by longitudinal grooves 42 into three axialrows of teeth 40 ₁, 40 ₂, 40 ₃, in which the teeth of the outer toothingare arranged in each case in the same shape as and parallel to eachother. The locking bushings 34 (see FIGS. 2 and 6) comprise acomplementary inner toothing, which is also divided by longitudinalgrooves 44 ₁, 44 ₂, 44 ₃, into three axial rows of teeth 46 ₁, 46 ₂, 46₃, in which the teeth of the inner toothing are arranged in each case inthe same shape as and parallel to each other. The longitudinal grooves42 _(i) in the outer toothing of the connecting rod 22 are somewhatwider than the teeth 46 _(i) of the locking bushing 34, and thelongitudinal grooves 44 _(i) in the inner toothing of the lockingbushing 34 are somewhat wider than the teeth 40 _(i) of the connectingrod 22.

In a first angular position of the connecting rod, shown in FIG. 3, theteeth 46 _(i) of the outer toothing of the rack 22 lie in thelongitudinal grooves 44 _(i) of the locking bushing 34. In this angularposition the connecting rod 22 can be pushed through the locking bushing34, the teeth 40 _(i) of the outer toothing being guided by thelongitudinal grooves 44 _(i) of the inner toothing, and the teeth 46_(i) of the inner toothing being guided by the longitudinal grooves 42_(i) of the outer toothing. FIG. 7 shows, in a cross section along thesection line A—A of FIG. 3, the teeth of the inner toothing in thelongitudinal grooves of the outer toothing.

In a second angular position—see FIG. 4—after the turning of theconnecting rod 22 by an angle γ=180°/n (n=number of longitudinal groovesor of rows of teeth), the teeth 46 _(i) of the connecting rod 22 arelocated axially between the teeth 40 _(i) of the locking bushing 34. Inthis second angular position, therefore, the rows of teeth of the innertoothing engage into the rows of teeth of the outer toothing for thetransmission of the necessary closing force.

FIG. 8 shows a section along the section line B—B of FIG. 4. It can beseen that the teeth of the outer and inner toothings have a trapezoidalcross section. The toothings can be developed helically, i.e. the teethare arranged along a helical line, and the toothings accordingly form athread having a pitch P. The toothings can, however, also be annular,i.e. the teeth can form parallel rings which are arranged in each caseat a distance P apart (also called pitch P).

In order that the outer toothing can engage into the inner toothing uponthe turning of the connecting rod 22, the teeth 46 _(i) of the innertoothing must, of course, be axially between the teeth 40 _(i) of theinner toothing in the first angular position of the connecting rod. Inorder that small errors in position of the movable closure plate do notprevent engagement of the inner toothing into the outer toothing, arelatively large axial flank clearance is desired between the innertoothing and the outer toothing.

From FIG. 8 it can be seen that P=2D+S

in which:

 P=pitch;

D=average tooth width;

S=axial flank clearance.

In practice, it has proven to be advantageous for S to be equal to 0.5D,and therefore P to be equal to 2.5D.

The extremely high closing force is transmitted by four connecting rods22 via the locking bushings 34 to the movable closure plate 20. Inaddition, the frequency in actual practice of the closings and openingsis very high. In other words, the material is subjected to substantialchanges in stress with high frequency. In this way, permanentdeformations can occur which impair the operation of the locking device.In order to reduce the negative effects of such permanent deformationson the locking function, the following measures can advantageously betaken:

a) The locking bushings 34 are so fastened to the movable closure platethat they are under tensile stress upon transmission of the closingforce. In this way, the result is obtained that both the connecting rods22 and the locking bushings 34 are lengthened in the same direction,i.e. are uniformly deformed.

b) The cross sections of the locking bushings 34 and of the connectingrods 22 should be so developed that they are subjected to approximatelythe same tensile stresses upon transmission of the closing force.

c) With the same tooth geometry, the base of the teeth of the outertoothing should be approximately equal to the base of the teeth of theinner toothing, so that the stress maxima at these critical places areapproximately the same. This means, for instance, that the arc length(in degrees) of the teeth of the outer toothing is greater than the arclength of the teeth of the inner toothing.

d) The teeth of the outer toothing should be of greater hardness thanthe teeth of the inner toothing. In this connection, the flank surfaceof the teeth of the outer toothing should also be larger than the flanksurface of the teeth of the inner toothing so that an imprint of theteeth of the outer toothing on the softer teeth of the inner toothing isavoided.

e) The elastic limit of the connecting rods 22 should be about 20%greater than the elastic limit of the locking bushings 34. In this way,in combination with measures b) and c), the result is obtained thatplastic deformations upon overloading occur in particular on the lockingbushings 34 and less so on the connecting rods 22. Plastic deformationson the connecting rods 22 are far more disturbing, since they namely arelocally limited and therefore destroy the axial homogeneity of the outertoothing in the rod section A, which can lead to inaccuracies in thepositioning of the closure plate if molds of different size are used.Furthermore, the replacement of the connecting rods 22 is far moreexpensive than the replacement of the locking bushings 34.

It should be pointed that measures a), b) and c) of the aboveenumeration have advantageous effects on the distribution of the tensileforce also in the normal case of elastic deformation. The elasticdeformation of the locking bushings and the elastic deformation of theconnecting rods are caused by these measures to take place in the samedirection and be of the same order of magnitude, so that the tensileforce to be transmitted is distributed over all interengaged teeth ofthe outer and inner toothings.

In the description of FIG. 8, it was pointed out that substantial flankclearance has the advantage that small inaccuracies in the positioningof the closure plate 20 by the displacement cylinders 25 do not preventengagement of the outer toothing of the connecting rods 22 into theinner toothing of the locking bushings 34. However, a substantial axialflank clearance also has essential disadvantages. First of all, thestroke of the piston 28 increases with the flank clearance S, as aresult of which the consumption of oil and energy by the force cylindersbecomes greater. Secondly, the connecting rods 22 are initially imparteda high acceleration when acted on with pressure by the force cylinders,so that the teeth of the outer toothing strike strongly against theteeth of the inner toothing. For this reason, it is advantageous toprovide a device which reduces or eliminates the flank clearance in thedirection of the transmission of force.

One extremely advantageous development of this device is described withreference to FIG. 9. The connecting rod 22 is coupled turnably to theannular piston 28 via a thread 50 (hereinafter called the screw thread50). The latter is secured against turning, for instance by a spline 52in a spline groove. If, accordingly, the connecting rod 22 is turned byan angle γ, it experiences an advance in the annular piston 28 of:

X=(γ/360°)P′

in which P′ is the pitch of the screw thread 50.

The turning of the connecting rod 22 is effected via a turning device 54which is arranged in the extension of the force cylinder 26. Thisturning device 54 comprises a housing 56 which is, for instance, flangedonto the housing of the force cylinder 26. A toothed bushing 58 isarranged, turnable, in two ball bearings 60, 62, within the housing 56.The toothed bushing 58 is placed on the end of the connecting rod 22 andso attached to this end via a tooth or spline-shaft connection that amoment of rotation is transmitted in form-locked manner, but at the sametime an axial displacement of the connecting rod 22 in the toothedbushing 58 is possible. The angular position of the toothed bushing 58,and thus the angular position of the connecting rod 22, can be adjustedvia an actuator 70 (see also FIG. 10) which engages into the outertoothing 64 of the toothed bushing 58. It should be pointed out that apure moment of rotation is transmitted to the connecting rod 22. Allradial forces which act on the toothed bushing 58 are transmitteddirectly by the two ball bearings' 60, 62 to the housing 56. In this wayassurance is had that the screw thread 50 is not stressed further bysetting forces.

FIG. 10 shows an advantageous development of an actuator 70 for twotoothed bushings 58′ and 58″ each. This actuator 70 comprises a rack 72,the toothing 74′ of which can engage into the toothed bushing 58′ andthe toothing 74″ of which can engage into the toothed bushing 58″. Therack 72 is arranged in a housing tube 73. In each end of the rack 72there is a cylindrical bore 76′, 76″. Pistons 78′, 78″ are introduced,sealed-off, into the respective cylinder bores 76′, 76″. These pistonsare advantageously developed as plunger pistons. The pistons 78′, 78″are flanged axially onto the two ends of the housing tube 73. The rack72 is displaceable back and forth axially in the housing tube 73 betweenthe two pistons 78′, 78″. In FIG. 10, the rack is shown resting againstthe left piston 78′; arrow 80 indicates the possible stroke of the rack72 in the direction of the right piston 78″.

Both pistons 78′, 78″, which also have a guide function for the rack 72,have an axial connecting channel 82′, 82″ for a pressure fluid. Viathese connecting channels 82′, 82″, the cylinder bores 76′, 76″ can beacted on optionally by the pressure fluid behind the pistons 78′, 78″ sothat two oppositely acting pressure cylinders are developed for thedisplacement of the rack 72. It should be noted that these two pressurecylinders are arranged directly above the toothed bushings 58′, 58″. Inthis way, the structural length of the actuator is reduced to a minimum.It should also be noted that the actuators are so designed that theconnecting rods 22 are turnable in each case from the first angularposition to the right and to the left by an angle γ.

From FIGS. 2 and 9 it can be seen that an axial guide device 90 isprovided for each of the connecting rods 22 on the movable closure plate20. This guide device 90 comprises a housing 92 which is rigidlyconnected to the movable closure plate 20. Within this housing a ring 94is rotatably mounted. This ring 94 serves as mount for three slide shoes96. As can be noted from FIG. 2, the three longitudinal grooves 42 onthe connecting rods 22 are developed as guide surfaces for these slideshoes 96 and are extended over the rod section A, up to the end plate24. The connecting rods 22 are accordingly always centered in thelocking bushings 34. If the connecting rod 22 is turned for lockingaround its axis, the ring 94 turns with the slide shoes 96.

FIGS. 11 to 14 show various embodiments of a connecting rod 22 as wellas various arrangements of the slide shoes 96 and embodiments of theguide surfaces for the slide shoes 96. In accordance with the embodimentshown in FIG. 11, the connecting rod 22 comprises two longitudinalgrooves 42 ₁, 42 ₂which divide the outer toothing into two rows of teeth40 ₁, 40 ₂. The slide shoes 96 ₁, 96 ₂ are guided in guide channels inthe longitudinal grooves 42 ₁, 42 ₂. Corresponding to the embodimentshown in FIG. 12, the connecting rod comprises three longitudinalgrooves 42 ₁, 42 ₂, 42 ₃ which divide the outer toothing into three rowsof teeth 40 ₁, 40 ₂, 40 ₃. The guide surfaces for the slide shoes 96 ₁,96 ₂, 96 ₃ are developed as flat surfaces which are at an angle of 120°to each other. The embodiment in accordance with FIG. 13 differs fromthe embodiment of FIG. 12 in the manner that the connecting rod 22 hasfour guide surfaces 42 ₁, 42 ₂, 42 ₃, 42 ₄ which are at an angle of 90°to each other. In accordance with FIG. 14, the outer toothing is dividedby six longitudinal grooves into six rows of teeth; however only everysecond longitudinal groove is developed as guide surface for a slideshoe 96 ₁, 96 ₂, 96 ₃. It is obvious that larger closing units requiremore rows of teeth and slide shoes than smaller closing units do.

On the basis of FIGS. 15 to 20, the design of the pitch of the thread 50for the taking up of the axial flank clearance S will be explained infurther detail. These figures show in each case a 180° development ofthe outer and inner toothings of FIGS. 3 and 4. There can be noted twoof the three rows of teeth of the inner toothing of the locking bushingand one of the three rows of teeth of the outer toothing of theconnecting rod. The teeth of the outer toothing are shown hatched. Thefollowing designations are used in the drawings:

P: pitch of the outer toothing on the connecting rod 22, or of the innertoothing on the locking bushing 34;

D: average tooth width;

S: axial flank clearance between inner toothing and outer toothing;

P′: pitch of the screw thread 50 between connecting rod 22 and piston28.

FIGS. 15, 17 and 19 show the position of the outer toothing before andafter a 60° rotation of the connecting rod in counterclockwisedirection. Before the 60° rotation, the teeth of the outer toothing liein a first angular position in the longitudinal grooves between the rowsof teeth of the inner toothing. After this 60° rotation incounterclockwise direction, the teeth of the outer toothing lie in thesecond angular position with their left flanks against the teeth of theinner toothing and can transmit a force to the left without play fromthe connecting rod to the locking bushing. FIGS. 16, 18, and 20 show theposition of the outer toothing before and after a rotation of theconnecting rod by an angle of 60° to the right. Before the 60° rotation,the teeth of the outer toothing lie in a first angular position in thelongitudinal grooves between the rows of teeth of the inner toothing.After this 60° rotation in clockwise direction, the teeth of the outertoothing lie in a second angular position with their right flank againstthe teeth of the inner toothing and can without play transmit a force tothe right from the connecting rod to the locking bushing. For thedesigning of the pitch of the thread 50 for the taking up of the axialflank clearance S, it is assumed that, in the starting position, beforethe turning of the connecting rod, the rows of teeth of the outertoothing are in each case angularly precisely in the center between therows of teeth of the inner toothing, and that the axial flank clearanceS between inner toothing and outer toothing is distributed equally onboth sides.

In the general case, the pitch of the screw thread is so designed thatby turning the connecting rod from the first angular position into thesecond angular position, the existing flank clearance S between innerand outer toothings is distributed on one side in such a manner that noessential flank clearance is present any longer between the tooth flankswhich are to transmit force.

FIGS. 15 and 16 refer to the case of an annular toothing. The pitch ofthe screw thread 50 is so designed that, by turning the connecting rodfrom the first angular position into the second angular position, theadvance of the connecting rod corresponds approximately to half of theflank clearance S between inner and outer toothings, i.e.:

P′/6=0.5S or P′=3S;

for the special case that S=0.5D, i.e. S=P/5, we have accordingly:

P′=0.6P.

FIGS. 17 and 18 refer to the case of a helical toothing which ascends indirection of rotation of the locking bushing in the direction of theforce to be transmitted. If it is assumed that the pitch P′ of the screwthread also ascends in the direction of rotation of the locking bushingin the direction of the force to be transmitted, then the advance X ofthe connecting rod must correspond approximately to half of the flankclearance S between inner and outer toothings plus one-sixth of thepitch P of the toothing, i.e.:

P′/6=0.5S+P/6 or P′=3S+P.

For the special case of S=P/5, i.e. S=0.5D, we have accordingly:

P′=1.6P.

FIGS. 19 and 20 refer to the case of a helical toothing which has anegative pitch in the direction of turning of the connecting rod in thedirection of the force to be transmitted. Furthermore, in FIGS. 19 and20, the toothing is developed with a double thread, i.e. S=0.5P−2D. Ifone proceeds from the basis that the pitch P′ of the screw thread musthave a positive pitch, then the advance X of the locking bushing mustcorrespond approximately to half of the flank clearance S between innerand outer toothings minus one-sixth of the pitch P of the toothing,i.e.:

P′/6=0.5S−P/6 or P′=3S−P;

for the special case of S=P/10, i.e. D=P/5, we have:

P′=−0.7P.

The minus sign in this case means that the screw thread 50 must alsohave a negative pitch.

FIG. 21 shows a block diagram of the hydraulic control of the closingunit 10. A 4/3-way proportional valve 100 has its first work outlet Aconnected in each case via a 2/2-way switch valve 102 ₁, 102 ₂, etc. tothe first pressure chamber 30 of each of the four force cylinders 28.The 4/3-way proportional valve 100, which is controlled by a controller103, controls by its work outlet A, upon the closing process, theclosing pressure in the first pressure chamber 30 of the four forcecylinders 26 as a function of a predetermined closing force 104. Thework outlet B of the 4/3-way proportional valve 100 is connecteddirectly to the second pressure chamber 31 of each of the four forcecylinders 26.

The closure plate 20 is provided with a position sensor 105 which isconnected to an axis control 106. A position sensor 108 is associatedwith the piston 28 of each force cylinder 26. The output signals S1, S2,S3, S4 of these position sensors 108 are also input values of the axiscontrol 106. The reference numeral 110 indicates an input unit for thelength “1” of the injection mold, i.e. the axial distance betweenclosure plate 20 and injection plate 14. This length “1” is set by theaxis control 106 via its output 112, the latter controlling the twodisplacement cylinders 25.

Before the turning of the connecting rod 22 from the first angularposition into the second angular position, the teeth of the outertoothing should be positioned precisely axially between the teeth of theinner toothing of the locking bushing 34 in order to permit properengagement of the outer toothing into the inner toothing upon theturning of the connecting rod 22 into the second angular position. Inorder to make this axial positioning of the toothings possibleindependent of the length “1” set, the position of rest of the piston28, as a function of the length “1” set, is established hydraulicallywithin a range [−0.5P; +0.5P] around a predetermined reference position.In other words, the connecting rod 22 is displaced axially, relative toa reference point, by an amount y, in which connection −0.5P<y<+0.5P.All actual positions of the four pistons 28 are compared for thispurpose in the axis control 106 with the calculated desired position.The axis control 106, via the outputs V11, V12, V13, V14 controls the2/2-way switch valves. If the measured actual position of a piston 28corresponds to the predetermined desired position, the corresponding2/2-way valve 102 is closed. This control permits, at little expense, acontinuous adjustment of the length “1”, regardless of the pitch of theinner and outer toothings. It is pointed out that only one proportionalvalve is used for the control described above.

What is claimed is:
 1. A closing unit of a molding machine having (i)first and second platens which act as clamping plates for a mold, (ii) adisplacement device for moving said first and second platens relative toeach other, and (iii) a plurality of hydraulic force cylinders on saidfirst platen to produce a closing force, each said force cylinder havinga piston connected to a connecting rod for transmitting said closingforce to said second platen, comprising: an actuator associated withsaid first platen for axially rotating said connecting rods betweenfirst and second angular positions; first locking means along saidconnecting rods; and second locking means on said second platen, saidfirst and second locking means registering with each other in such amanner that in said first angular position said first and second lockingmeans permit an axial displacement of said second platen relative tosaid connecting rods and in said second angular position, said firstlocking means engages said second locking means for the transmission ofsaid closing force.
 2. A closing unit according to claim 1, wherein:said first locking means includes at least two rows of outwardlyextending teeth on said connecting rod separated by a longitudinallyextending groove; said second locking means includes a locking bushinghaving at least two rows of inwardly extending teeth separated by alongitudinally extending groove; and wherein said first angularposition, said rows of outwardly and inwardly extending teeth arerelatively axially movable respectively along said longitudinal groovesin said second and said first locking means to permit axial displacementof said second platen relative to said connecting rods and, in saidsecond angular position, said inwardly extending teeth engage saidoutwardly extending teeth to transmit said closing force.
 3. A closingunit according to claim 2, wherein an axial flank clearance whichcorresponds approximately to half of an average width of a tooth ispresent between said outwardly and said inwardly extending teeth.
 4. Aclosing unit according to any one of claims 2 or 3, wherein saidinwardly and said outwardly extending teeth are arranged annularly.
 5. Aclosing unit according to any one of claims 2 or 3, wherein saidinwardly and said outwardly extending teeth are arranged helically.
 6. Aclosing unit according to any one of claims 2 or 3, wherein saidinwardly and said outwardly extending teeth have a trapezoidal crosssection.
 7. A closing unit according to any one of claims 2 or 3,wherein a piston of said hydraulic force cylinder is connected via ascrew thread to said connecting rod, said piston being secured againstrotation, so that by the turning of the connecting rod from said firstangular position to said second angular position, said connecting rodadvances relative to said piston.
 8. A closing unit according to claim7, wherein said actuator can turn said connecting rods either way fromsaid first angular position, to advance or retract said connecting rodsrelative to said piston.
 9. A closing unit according to claim 7, whereinthe pitch of said screw thread is such that by turning said connectingrod from said first angular position to said second angular position,said existing flank clearance between said inwardly and outwardlyextending teeth is substantially eliminated between respective flanks ofsaid inwardly and said outwardly extending teeth which are to transmitsaid closing force.
 10. A closing unit according to claim 7, wherein thepitch of the screw thread is designed in such a manner that turning theconnecting rod from the first angular position into the second angularposition causes the connecting rod to advance by an amount whichcorresponds to approximately one half of said flank clearance.
 11. Aclosing unit according to claim 3, wherein said inwardly and saidoutwardly extending teeth form a thread having a pitch which causes saidflank clearance to be substantially taken up as said connecting rod isrotated between said first and second angular positions.
 12. A closingunit according to claim 11, wherein said inwardly and outwardlyextending teeth form a thread, and wherein turning the connecting rod ineither direction from said angular position into said second angularposition, causes said flank clearance to be substantially taken up. 13.A closing unit according to claim 12, wherein said thread is developedas a double thread.
 14. A closing unit according to claim 2, whereinsaid locking bushing is so arranged on said second platen that it isunder tensile stress upon transmission of said closing force.
 15. Aclosing unit according to claim 14, wherein said connecting rod has anelastic limit which is about 20% greater than that of said lockingbushing, and wherein said connecting rod and locking bushing are sodimensioned as to have average tensile stresses respectively thereuponduring transmission of said closing force which are approximately equal.16. A closing unit according to any one of claims 14 or 15, wherein thelength of a base of said inwardly extending teeth is equal to the lengthof a base of said outwardly extending teeth.
 17. A closing unitaccording to any one of claims 14 or 15, wherein said inwardly extendingteeth have a smaller flank surface than said outwardly extending teethand said outwardly extending teeth are of a greater hardness than saidinwardly extending teeth.
 18. A closing unit according to claim 2,further comprising a first platen to which said connecting rods aremounted and with respect to which said connecting rods are rotatable andaxially displaceable.
 19. A closing unit according to claim 2, whereinsaid second platen has turnably mounted slide shoes which act as aradial guide for said connecting rods, said longitudinal grooves betweensaid rows of outwardly extending teeth forming guide surfaces for saidslide shoes, said guide surfaces extending longitudinally beyond saidoutwardly extending teeth, and said slide shoes being turnable with saidconnecting rod.
 20. A closing unit according to claim 1, wherein eachsaid actuator has a common rack for simultaneously rotating two of saidlocking bushings.
 21. A closing unit according to claim 20, wherein saidrack has a cylinder bore at each end, a fixed piston sealingly engageseach of said two cylinder bores so that said rack is displaceableaxially back and forth between said two fixed pistons, and pressurizedfluid may be introduced between said pistons and said cylinder bores sothat two oppositely directed pressure cylinders are formed for thedisplacement of the rack.
 22. A closing unit according to claim 21,wherein each said piston has an axial channel extending therethrough foradmitting said pressurized liquid between said cylinder bore.
 23. Aclosing unit according to any one of claims 20, 21, or 22, furthercomprising a drive bushing with outer toothing which meshes with saidrack, said drive bushing being rotatably mounted in a housing, having anopening for the axial introduction of said locking bushing, and couplingmeans for imparting rotation from said drive bushing to said lockingbushing, said coupling means further permitting axial displacement ofsaid connecting rod in said drive bushing.
 24. A closing unit accordingto claim 23, wherein said coupling means comprises a spline-shaftconnection.
 25. A closing unit according to claim 1, wherein said forcecylinders comprise double-acting annular pressure cylinders each havinga first pressure chamber for locking said injection mold and a secondpressure chamber for opening said injection mold.
 26. A closing unitaccording to claim 25, further comprising a 4/3-way proportional valvewith a first work outlet and a second work outlet, said first workoutlet being connected via respective 2/2-way valves with said firstpressure chambers of said force cylinders and said second work outletbeing connected with said second pressure chambers of said forcecylinders.
 27. A closing unit according to claim 26, further comprisinga position sensor for sensing the position of said piston of each saidforce cylinder, and a control unit which closes each said 2/2-way valvewhen the position of said piston corresponds to a predeterminedposition.
 28. A closing unit according to claim 27, further comprising acontrol unit for axially positioning said pistons of said forcecylinders in a position of rest in which, upon the turning of saidconnecting rods from said first angular position into said secondangular position, said inwardly extending teeth lie axially between saidoutwardly extending teeth.
 29. A closing unit according to claim 28,further comprising a position sensor for sensing the position of saidfirst platen, a position sensor for sensing the position of said pistonof each said force cylinder, a calculating unit for calculating saidposition of rest of said pistons as a function of the position of saidfirst platen, in such a manner that said teeth of said connecting rodslie axially between said teeth of said locking bushings, beforeengagement thereof, and said control unit receives input from saidposition sensors of said pistons for positioning each piston into saidposition of rest.